(1) Field of the Invention
The present invention relates to a method of producing a semiconductor device, more particularly to a method of forming a shallow n-type region in a semiconductor device, such as a bipolar transistor, a metaloxide semiconductor field-effect transistor (MOS FET), or a diode. The present invention can be suitably applied to form a shallow emitter region of an npn-type bipolar transistor.
(2) Description of the Prior Art
Integrated circuits (IC's) comprising bipolar transistors or MOS FET's have recently been made denser by miniaturization of individual elements of the IC's. The final aim of all this, of course, is to further increase the speed of operation and improve the frequency characteristics of the IC and the individual transistors.
In the case of a bipolar transistor, narrowing of the width of the base region between an emitter region and a collector region is most effective for improving the speed of operation and frequency characteristics. Therefore, it is important to form a shallow junction. Various methods for forming a shallow junction have been proposed. One effective method of formation which has recently been used frequently includes: forming a polycrystalline silicon film on an exposed p-type base region of a silicon substrate and on a silicon oxide film selectively formed on the base region. Then, arsenic ions and phosphorus ions are doped into the polycrystalline silicon film by an ion-implantation method. Finally the arsenic ions and phsophorus ions are thermally diffused into the base region out of the polycrystalline silicon film by heat-treatment to form a shallow n-type emitter region.
In this case, the formation of the polycrystalline silicon film prevents the silicon substrate from being damaged by the ion-implantation. The use of both arsenic and phosphorus as the n-type impurity results in less crystal lattice strain since the larger atomic radius of arsenic compared with that of silicon is compensated by the smaller atomic radius of phosphorus compared with that of silicon. More specifically, when impurities are diffused into a single crystalline substrate, the diffused impurities cause crystal lattice strain. Diffusion of either arsenic or phosphorus alone increases the crystal lattice strain and, under certain circumstances, generates abnormal diffusion resulting in an emitter-collector short-circuit.
The diffusion depth of the n-type impurities determines the dimensions of the n-type emitter region and, in turn, the width of the base region. Since the diffusion coefficient of phosphorus is larger than that of arsenic, it is the diffusion depth of phosphorus that essentially determines the emitter region. However, it is more difficult to control the diffusion depth of phosphorus than arsenic or, in the case of the p-type impurity for the base region, boron. Therefore, this method of formation is difficult in that it is necessary to exactly adjust the emitter depth. It is possible to try doping phosphorus ions shallowly into the polycrystalline silicon film by decreasing the implantation energy of the phosphorus ions, but such shallow implantation is difficult because of the implantation apparatus.